The present invention relates to a method for evaluating particle concentrations, such as surface contamination or atomic contamination and especially boron or phosphorus, in the atmosphere of a clean room or a machine mini-environment.
A clean room is a work area with controlled temperature, humidity and particle concentration to protect sensitive equipment or products from contamination. Many different organizations such as medical groups or integrated circuit manufacturers require facilities that include high quality clean rooms. These clean rooms are equipped with various expensive and complicated air supply cleaning systems that provide the rooms with dust-free filtered air. Nevertheless, in typical clean rooms used to manufacture semiconductor wafers, due to system leakage and inconsistent air filtering, a certain small amount of contaminants such as boron and phosphorus remains in or is added to the clean room atmosphere, and this can lead to contamination of the wafers. For example, boron or phosphorus contamination on a surface of a processed silicon wafer can amount to 1012 atoms per cm2. These particles can diffuse into the silicon wafer surface during thermal treatment leading to a change of the dopant concentration in this region which affects the characteristic of the processed chips. Therefore, it is necessary to be aware of the particle concentration of the clean room.
Conventional technology used to monitor boron or phosphorus levels in a clean room environment operate by funneling room air through a liquid to form air bubbles, wherein part of the boron or phosphorus of the air bubbles dissolves in the liquid. After a relatively long period of time, which may be on the order of a day, the liquid picks up a large enough amount of boron or phosphorus that is adequate for analyzing. The boron or phosphorus content of the liquid is then evaluated, for example, by using a spectroscopy based analysis such as Atomic Spectroscopy or ICPMS.
This method is a relatively imprecise way to tell the real boron or phosphorus concentration of the air, because only an indefinite part of the actual boron or phosphorus content actually dissolves in the liquid. Most air samples containing boron or phosphorus escapes with the air bubbles from the liquid before dissolving. A further disadvantage is that this method takes a relatively long period of time to obtain results, and these results provide only a mean value of the boron or phosphorus of the clean room. Thus, improvements are needed in this area.